1. Field of the Invention
This invention relates to foamable organosiloxane compositions. More particularly, this invention relates to foamable organosiloxane compositions containing an additive that imparts thixotropy to the compositions without adversely affecting other properties of the compositions or cured foams prepared from the compositions. The additive is particularly useful for preparing low density foams of small cross-sectional area.
2. Background Information
Foamable compositions comprising a curable liquid polyorganosiloxane, an organohydrogensiloxane, a hydroxyl-containing compound and a platinum group metal or a compound thereof as the foaming and curing catalyst have been described in the patent and journal literature. Relevant patents include U.S. Pat. Nos. 3,923,705, which issued on Dec. 2, 1975; 4,026,843, which issued on May 31, 1977; 4,189,545, which issued on Feb. 19, 1980; 4,590,222, which issued on May 20, 1986; and 4,599,367, which issued on Jul. 8, 1986, and 5,252,627, which issued on Oct. 12, 1993.
The curable polyorganosiloxane used to prepare the foam contains at least two alkenyl or silanol groups per molecule. If the polyorganosiloxane does not contain silanol groups a low molecular weight hydroxyl compound such as an alcohol, silanol or siloxanol is included in the reaction mixture to generate the hydrogen required for foaming of the composition.
For certain end use applications such as gaskets it is desirable that the cured foam have a density less than 20 lb/ft.sup.3 (0.32 g/cc). The viscosity of the curable organosiloxane compositions used to prepare these foams is typically from 20 to 70 Pa.s. These composition flow to the extent that it is difficult to prepare three dimensional objects in the absence of dams or other barriers to prevent flowing of the composition prior to curing.
A variety of additives have been used to impart thixotropic character to liquid compositions, thereby reducing the tendency of the these compositions to flow in the absence of shearing forces. These additives include finely divided forms of untreated silica, silica that has been treated with silanes or other liquid hydroxyl-containing organosilicon compounds, asbestos fibers, carbon black and other forms of finely divided carbon, clays that have been treated with quaternary ammonium compounds, phthalocyanine and other organic pigments, finely divided metal carbonates, oxides, hydroxides and silicates, lithium hydroxystearate, soaps of aluminum and other metals, cellulose esters, colloidal forms of alkali or alkaline earth metal aluminum silicates, and hydrogenated glycerides of unsaturated fatty acids, such as those present in castor oil. These thixotropic agents are disclosed in one or more of the following references:
European Published Application No. 0198536, published on May 16, 1984; PA1 U.S. Pat. No. 4,497,919, published Feb.5, 1985; PA1 Japanese Laid Open Application No. 2/38,309, published Feb. 7, 1990; and PA1 The third edition of the Kirk Othmer Encyclopedia of Chemical Technology, volume 19, page 530 and volume 20, page 262. PA1 A. a liquid polyorganosiloxane containing at least two reactive groups per molecule selected from the group consisting of silanol groups and alkenyl radicals; PA1 B. an amount sufficient to foam and cure said composition of an organohydrogensiloxane containing at least three silicon-bonded hydrogen atoms per molecule wherein there is no more than one of said hydrogen atoms per silicon atom; PA1 C. a compound containing at least one carbinol or silanol group per molecule; and PA1 D. as the catalyst, an amount of a metal from the platinum group of the periodic table or a compound of said metal sufficient to promote foaming and curing of said composition, wherein the improvement comprises the presence in said composition of a thixotropic additive in an amount sufficient to prevent substantial flowing of said composition prior to curing, said additive consisting essentially of the hydrogenated glyceride of at least one ethylenically unsaturated carboxylic acid containing from 12 to 20 carbon atoms.
The present inventors discovered that some of the known thixotropic agents, particularly treated and untreated forms of finely divided silicas, adversely affect the processability of foamable organosiloxane compositions in addition to altering the texture, density and other physical properties of the cured foam to the extent that the foam is no longer suitable for its intended end use.
One objective of this invention is to provide foamable compositions that can be formed into three dimensional low density cured articles by casting or extrusion on to unobstructed surfaces.